Protective system for electrical discharge machining power supply circuit

ABSTRACT

A circuit is provided to sense an electrical parameter of the gap to determine whether electrical discharge machining is progressing properly. A gap parameter such as gap voltage or a voltage directly related to such parameter is compared to a variable reference voltage to provide reduction or interruption of machining power pulses. A control is included which conjointly controls frequency of machining pulses and the magnitude of the variable reference voltage.

United States Patent Sennowitz 5] Mar. 14, 1972 [54] PROTECTIVE SYSTEMFOR ELECTRICAL DISCHARGE MACHINING POWER SUPPLY CIRCUIT [72] Inventor:Kurt II. Sennowitz, Royal Oak, Mich.

[73] Assignee: Elox Inc., Davidson, NC.

[22] Filed: Dec. 28, 1970 [211 App]. No.: 102,009

Related US. Application Data [63] Continuation-impart of Ser. No.851,882, Aug. 21, 1969, Pat. No. 3,558,428, Continuation-impart of Ser.No. 851,952, Aug. 21, 1969.

[52] US. Cl. /69C [51] Int. Cl 1123b H08 [58] Field ofSearch ..2l9/69 C[56] Relerences Cited UNITED STATES PATENTS 3,158,728 11/1964 Webb..2l9/69 0 Primary Examiner-R. F. Staubly Attorney-Hank's, Gifford andPatalidis [57] ABSTRACT A circuit is provided to sense an electricalparameter of the gap to detennine whether electrical discharge machiningis progressing properly. A gap parameter such as gap voltage or avoltage directly related to such parameter is compared to a variablereference voltage to provide reduction or interruption of machiningpower pulses. A control is included which conjointly controls frequencyof machining pulses and the magnitude of the variable reference voltage.

10 Claims, 2 Drawing Figures A; iz

PROTECTIVE SYSTEM FOR ELECTRICAL DISCHARGE MACHINING POWER SUPPLYCIRCUIT CROSS REFERENCE TO RELATED APPLICATIONS This application is acontinuation-in-part of my copending application, Ser. No. 851,882 filedon Aug. 21, 1969 now US. Pat. No. 3,558,428, issued June 28, 1971, forGap Open Circuit Protective System for Electrical Discharge Machining,"and of my copending application, Ser. No. 851,952 filed on Aug. 21, 1969for Gap Short Circuit Protective System for Electrical DischargeMachining.

BACKGROUND OF THE INVENTION The field to which my invention relates isthat known as electrical discharge machining in which material isremoved from an electrically conductive workpiece by the action ofelectrical gap discharges between a tool electrode and the workpiece. Anelectrode or workpiece servo-feed system is used to provide relativemovement to maintain an optimum gap spacing between electrode andworkpiece as material is removed. A dielectric coolant is circulated andrecirculated through the gap during machining operation. For mostreliable and predictable results, a power supply circuit of theindependent pulse generator type is utilized to provide machining pulsesof precisely controllable frequency and on-off time.

During the machining operation, the gap may become bridged by workpieceparticles to cause a localized effect known as gap short circuitcondition. This condition is accompanied by excessive localized heatwhich tends to damage both electrode and workpiece unless correctiveaction is taken. Gap open circuit condition is similarly a problem.Voltage of an excessive level can contribute to excessive sludge buildupand DC arcing. This is critical at the beginning of a cut, particularlyif only a small portion of the electrode is initiating the cut, such aswould occur in narrow slot machining operations. The present applicationis directed to an improved protective circuit which controls machiningpower responsive to an abnormal gap condition such as those describedabove. The protective circuit includes a variable reference voltagewhich is both preset and selectively varied according to the frequencyof machining power pulses.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a combined schematic andblock diagrammatic showing a complete EDM protective circuit andincorporating my invention in that portion of the circuit related to gapshort circuit control; and

FIG. 2 is essentially similar to FIG. 1 but includes a somewhatdifferent gap open circuit control arrangement.

DESCRIPT ION Referring now to the FIG. 1 drawing, a main machining powersource 10 is shown connected with a pair of output stage transistors 12,12 and in series with the machining gap comprising a tool electrode 14and a workpiece 16. Gap current magnitude is controlled by variableresistors l8, l8. Rectifier 20 is included in series between the toolelectrode 14 and the output stage transistors 12, 12' to permit high gapvoltage operation without damage to those transistors. A diode 22 and acapacitor 24 are included to clamp the transistors l2, 12' to thepositive terminal of the DC supply 10 to eliminate high voltage spikes.

A multivibrator stage 26 and a plurality of successive driver stages 28,30 and 32 are used to control the conduction of the output stagetransistors 12, 12'. In the interest of simplification and brevity, thestages 28, 30 have been shown in block form. Included in themultivibrator 26 are a pair of transistors 34, 36, biased and crosscoupled for alternate operation in the astable multivibrator mode. Thetransistors 34 and 36 are connected respectively through their loadresistors 38 and 40 to the negative terminal of a DC source 42. A pairof crosscoupling capacitors 44, 46 are adjustable by ganged control andserve to connect collectors of the transistors 34, 36 each to theopposing transistor base. Variable capacitors 44, 46 may be embodied inthe form of a plurality of different magnitude capacitors selectivelyconnected in the circuit by a suitable tap switch. A pair ofcurrent-limiting resistors 48, 50 and a potentiometer 52 are included inthe multivibrator circuit with the multivibrator on-off time, and theresultant machining power pulse width, controlled by the setting of themovable contact of the potentiometer 52. A pair of blocking diodes 54,56 are connected as shown in circuit with the movable contact of thepotentiometer 52. The frequency of the multivibrator operation and henceof the machining power pulses furnished to the gap may be selectivelyaltered by changing the values of the capacitors 44, 46 by a gangedswitch control as is indicated by the dash lines between thosecapacitors. A further dash line is shown between the capacitor 46 andthe tap switch 58 to indicate automatic control of reference voltagelevel responsive to change of multivibrator operating frequency. Thepulse output from the multivibrator 26 is suitably amplified andresquared through the intermediate drive stages 28, 30 and 32 to renderthe output transistors 12, 12' alternately conductive and nonconductive,and thus provide machining pulses to the gap. Resistors 60, 62, 64protect the base emitter junction of their respective transistors fromexcess turnofi voltage. While the circuit including the presentinvention employs transistors as electronic switches, the invention isnot so limited but, with proper redesign of the circuit and itscomponents by one skilled in the art, other electronic switches may besubstituted. By the term electronic switch is meant any electroniccontrol device having three or more electrodes comprising at least twopower electrodes acting to control current flow in the power circuit,the conductivity of the power circuit being controlled by a controlelectrode within the switch whereby the conductivity of the powercircuit is controlled statically or electrically without movement of anymechanical elements within the switch. Included within this definitionare vacuum tubes, transistors and other like devices.

The remainder of the circuit shown in FIG. 1 relates to the gapshort-circuitand gap open circuit-protective systems. With reference tothe former, a bias source 66 is included with the resistors 67, 68, 70connected thereto as a voltage divider. The sensing network connected tothe tool electrode 14 includes a diode 72 and a variable capacitor 76whose charge and discharge rate depends on the values of a resistor 78and the capacitor 76. The setting of the capacitor 76 is nonnally suchto provide normal cutoff delay during roughing, high current cutting andfast cutoff during finish low current cutting, especially when machiningis performed at high frequencies.

A separate sensing network is connected to the drive stage comprisingthe transistor 32. This network is designed to feed back, store andamplify a portion of the drive signal to provide pulse narrowingoperation on the multivibrator 26, particularly with response to controlof the conduction of the transistor 36 which is the on-time controltransistor.

Included in this network are a resistor 80, a diode 82 and a capacitor84. The drive signal taken from the emitter of the transistor 32 chargescapacitor 84 to the polarity shown. Resistor 85 with the referencevoltage level across the capacitor 84 determines the charge anddischarge levels and the time constant. The capacitor 84 is embodied asa variable magnitude capacitor to permit adjustment for the desired gapcurrent during a gap short circuit condition or before the gap actuallyshorts, depending on the reference voltage adjustment. A load resistor87 is connected between the emitter of transistor 32 and a positivevoltage derived from the bias source 66.

Actual control over machining power pulse duration is exercised throughthe cutoff transistors 86 and 88. A resistor 89 is the drive limitingresistor for the transistor 86. A diode 90 protects the base to emitterjunction of the transistor 86 from excess turnoff voltage. A capacitor92 prevents pretriggering of the transistor 86 due to stray signals. Thetransistor 86 is maintained in a nonconductive state so long as thenegative electrode gap voltage on its base is more negative than thepreset reference voltage on variable resistor 94 applied through ahold-off diode 96 to its emitter. Resistor 98 is a part of the dividernetwork across the DC source 10. A zener diode 100 stabilizes thereference voltage, while a capacitor 102 provides a filter for thepreset reference voltage. As has already been indicated, a resistor 104is switched across the preset portion of variable resistor 94 conjointlywith each change made in frequency to the multivibrator 26 by adjustmentof the capacitors 44, 46. I have found that certain electrode materialscut a great deal more efficiently when higher cutoff reference voltageis used along with the higher frequency settings of the multivibrator26.

The short circuit cutoff transistor 86 has its collector connected tothe base of a transistor 88 through a drive limiting resistor 106. Adiode 108 is a protective diode for the base to emitter junction while acapacitor 110 filters out stray pickup to prevent pretriggering of thetransistor 88. A resistor 112 applies a positive bias signal across theemitter-base junction of the transistor 88 during its off-time. A diode114 isolates the cutoff circuit from the base of the transistor 36during normal machining operation. A resistor 116 is connected to thenegative terminal of DC source 42 and to the rectifier 114, transistor88 collectorjunction to insure normal multivibrator operation duringcutting.

The open circuit pulse narrowing circuit similarly uses the cutofftransistors 86 and 88 to exercise its current reducing function. Areference network is connected between the emitter of drive transistor32 and the plus arc terminal provided by the workpiece 16. This networkincludes a pair of series resistors 120 and 122 connected as a voltagedivider. The open circuit pulse narrowing circuit includes transistors124 and 126. A resistor 128 is the drive-limiting resistor for thetransistor 124. A rectifier 130 protects the transistor 124 base toemitter junction from excess turnoff voltage. A gap voltage sensingnetwork is connected between the lead to the electrode l4 and theemitter of the transistor 124. This network includes a diode 132 and acapacitor 134. It will be seen that the diode 132 senses and peak storesa portion of the gap voltage on the capacitor 134, to provide a gapvoltage signal on the emitter of the transistor 124. A resistor 136connected across the capacitor 134 determines the RC time constant ofoperation. The transistor 124 will conduct when the negative gap voltageat point B and on the emitter of the transistor 124 is more negativethan the preset pulsed voltage on the base of the transistor 124. Aresistor 138 limits the drive current to the base of the transistor 126through an isolating diode 140. A diode 142 protects the base to emitterjunction of the transistor 126 from excess turnoff voltage. A variableresistor 144 is connected to the positive terminal of bias source 66 andin series with a resistor 146. A resistor 148 is connected as thetransistor current limiter.

The circuit of FIG. 2 is substantially the same as FIG. 1 with respectto the EDM power supply circuit and that portion of the circuit relatedto gap short circuit condition current control. The gap circuit currentcontrol portion incorporates some differences. A resistor 150 and acapacitor 152 couple the drive signal during the drive transistor 32off-time through diode 154 to charge a capacitor 156 to the polarityshown. The capacitor 156 is shunted by a resistor 160. The drive signalwill also be passed to the gap through a diode 158, if the negative gapvoltage drops. A rectifier 158 clamps the drive signal to the minus gapvoltage. A drive current limiting resistor 162 and a series diode 164transfer the negative drive signal to the base of the transistor 166 toturn it on. A load resistor 168 limits the current of the transistor 166and places a positive signal on the base of transistor 86 to turn it on.In the conductive state of transistor 86, transistor 88 is turned on, ashas already been explained, to narrow the on-time of multivibrator 26,and hence limit the cutting current passed to the gap. The associatedcircuit of the transistor 166 is completed by a protective diode 170, afixed resistor 172, and a variable minal of DC bias source 66 to providea positive turnoff bias voltage to the base of the transistor 66.

DESCRIPTION OF OPERATION The description of operation will now be madewith special attention given to the gap short-circuitand gap opencircuitprotective systems and their functioning in conjunction withmultivibrator 26. During rough machining, the multivibrator 26 is presetthrough the potentiometer 52 to provide high current, long on-timepulses. The on-time of the machining pulses is controlled through theconduction of the transistor 36, while the off-time is controlledthrough the conduction of the transistor 34. During this mode ofoperation, the variable capacitor 76 is adjusted to give a relativelylarge cutoff delay period. This serves to improve cutting stability andimprove metal removal. During the initial open arc condition when theelectrode 14 is being advanced toward the workpiece 16 to start cuttingand during normal gap-cutting condition, the transistor 86 is in itsnonconducting state. The negative arc voltage taken from the electrode14 and peak stored on the capacitor 76 is applied to the base of thetransistor 86. Since the applied voltage is more negative than thereference voltage applied to its emitter, transistor 86 will stay off.During this period, the transistor 88 is also nonconducting, so that themultivibrator 26 is not affected in its operation.

When a gap short circuit actually occurs or is impending, the gapvoltage will drop below the preset reference voltage applied to theemitter of the transistor 86. When the base is rendered more positivethan the emitter, the transistor 86 will be rendered conductive. Thiswill apply a negative signal to the base of PNP-transistor 88 to turn iton. In its conductive state, the transistor 88 will place a positivesignal on the base of the transistor 36 through diode 114 to turn itoff. Since the transistor 36 controls machining pulse on-time, thepulses will be substantially narrowed or even completely interrupted. Anadjustable delay is exercised through the discharge of the capacitor 84through the resistor into the emitter of the transistor 86. This willhold the transistor 86 off for a short period during the initial part ofthe gap short circuit condition and delay the pulse narrowing effect.When roughing is completed, the multivibrator 26 is normally changed tooperate at a much higher frequency for finishing the cut. As capacitors44, 46 were first adjusted for the lower pulse frequencies, the resistor104 was switched across the preset portion of the variable resistor 94to provide a relatively low cutoff reference voltage. For the higherfrequency settings, the resistor 104 is disconnected from across thevariable resistor 94 to provide a relatively high reference voltage.During the finishing operation, little or no cutoff delay is desirableand the capacitor 84 is usually adjusted accordingly.

Open circuit condition operation will now be considered with referenceto the circuit of FIG. 1. On initial downfeed it is important that theon-time be shortened and the off-time be lengthened for a lowerfrequency than the normal cutting frequency. This serves not only toprotect the workpiece 16 and the electrode 14 from damage, but also toprotect the output stage switches 12, 12 from damage when current ispassed at higher voltage than normal cutting voltage. The transistor 124is normally nonconductive. The desired open are narrowing voltage rangeis set on the reference voltage divider network comprising the resistorsand 122 and applied to the base of the transistor 124. The gap voltageis sensed through the diode 132 and peak stored on the capacitor 134.When the negative gap voltage sensed is more negative than the presetpulsed reference voltage on its base, the transistor 124 will be turnedon. This will provide a negative voltage to the base of the transistor126 and turn it on to provide voltage to the base of the transistor 86which will in turn be rendered conductive. The pulse narrowing signalthen is passed to the multivibrator 26 in the manner previouslydescribed for short circuit narrowing. The open circuit pulse narrowingarrangement of FIG.

2 is similarly connected in the power supply circuit. The voltage signalprovided from the drive stage transistor 32 during its off-time iscoupled through the rectifier 154 to charge capacitor 156 to thepolarity shown. The transistor 166 is normally in a nonconducting state.The negative drive signal is transferred to the base of the transistor166 to turn it on when gap open circuit level is reached. The resistor172 and the variable resistor 174 are connected'to the plus terminal ofthe bias source 66 to hold a turnoff bias on the base of the transistor166. This bias on the base of the transistor 166 prevents oscillationsand false turn-on during normal cutting. The divider network resistors150, 160 and the capacitors 152, 156 values determine the open circuitpulse narrowing voltage across the gap together with the resistor 174bias setting required to turn on the transistor 166. Once the transistor166 is turned on, it will pass a positive signal to the base of thetransistor 86 to turn it on and initiate the pulse narrowing operationpreviously described on multivibrator 26. One feature of the FIG. 2 opencircuit pulse narrowing system is that it operates without taking anypower from the machining gap or output transistor stages. It actuallypasses current to the gap through the rectifier 158 during normalcutting conditions.

It will thus be seen that I have provided a novel and improved circuitfor abnormal gap voltage protection and one in which suitably adjuststhe cutoff level in accordance with the frequency at which EDM machiningis being conducted.

lclaim:

1. In an electrical discharge machining apparatus for machining anelectrically conductive workpiece by passing electrical dischargesacross a dielectric coolant filled gap between a tool electrode and theworkpiece, an electronic output switch and a power supply operativelyconnected across said gap, a pulser means operatively connected to saidswitch for operating it with pulses of selectively variable on-off timeand frequency, wherein the improvement comprises a circuit for reducingthe power content of said pulses responsive to gap abnormal condition,said circuit comprising a variable reference voltage network, a sensingnetwork operatively connected to said gap for sensing a parameterrepresentative of gap condition and providing a voltage outputrepresentative thereof, a cutoff means operatively connected to bothsaid networks and operable to reduce the on-time of said pulserresponsive to a predetermined difference between said voltages, andmeans operatively connected between said pulser means and said referencevoltage network for varying the magnitude of said reference voltage inresponse to the frequency preset on said pulser.

2. In an electrical discharge machining apparatus for machining anelectrically conductive workpiece by passing electrical dischargesacross a dielectric coolant filled gap between a tool electrode and theworkpiece, an electronic output switch and a power supply operativelyconnected across said gap, a pulser means operatively connected to saidswitch for operating it with pulses of selectively variable on-otf timeand frequency, wherein the improvement comprises a circuit for reducingthe power content of said pulses responsive to abnormal gap voltagecondition, said circuit including a variable reference voltage network,a sensing network coupled to one terminal of said gap for sensing gapvoltage and providing a voltage output representative thereof, a cutoffmeans operably connected to the output of both said networks andoperative to reduce the on-time of said pulser responsive to therelative values of said voltages, and means operably connected betweensaid pulser and said cutoff means for varying the magnitude of saidreference voltage in accordance with the frequency setting of saidpulser means.

3. In an electrical discharge machining apparatus for machining anelectrically conductive workpiece by passing electrical dischargesacross a dielectric coolant filled gap between a tool electrode and theworkpiece, an electronic output switch and a power operatively connectedto said gap, a presettable multivibrator operatively connected to saidelectronic switch for operating it with pulses of selectively variableand on-off time frequency, wherein the improvement comprises aprotective circuit for reducing the on-time of said pulses responsive togap abnormal condition, said circuit comprising a variable referencevoltage network, a sensing network coupled to one terminal of said gapfor providing an output responsive to gap voltage condition, a cutoffswitch operatively connected to both of said networks and having itsoutput operatively connected to said multivibrator to reduce itson-time, and a control means operatively connected between saidmultivibrator and said reference voltage network for varying the levelof said reference voltage network responsive to the frequency setting ofsaid multivibrator.

4. The combination as set forth in claim 3 wherein said control means isoperable to lower the level of said reference voltage at relatively lowfrequency settings of said multivibrator and to raise the level of saidreference voltage at relatively high frequency settings of saidmultivibrator.

5. The combination as set forth in claim 3 wherein said multivibratorcomprises an astable multivibrator including a variable capacitoroperatively connected thereto for selectively varying the frequency ofsaid multivibrator and wherein said reference voltage network includes avariable resistor, said control means comprising a ganged operatingmeans operatively connected between said variable capacitor and saidresistor.

6. The combination as set forth in claim 5 wherein said multivibratorincludes a pair of transistors rendered alternately conductive andnonconductive and wherein said cutofi' switch comprises a thirdtransistor having its control electrode and one of its principalelectrodes each connected to a different one of said networks, and itsother principal electrode coupled to the control electrode of one ofsaid multivibrator transistors for controlling its conduction.

7. In an electrical discharge machining apparatus for machining anelectrically conductive workpiece by passing electrical dischargesacross a dielectric coolant filled gap between a tool electrode and theworkpiece, an electronic output switch and a power supply operativelyconnected to said gap, a presettable multivibrator operatively connectedto said electronic switch for operating it with pulses of selectivelyvariable frequency and on-off time, wherein the improvement comprisesthe protective circuit for reducing the on-time of said pulsesresponsive to gap short circuit condition, said circuit comprising avariable reference voltage network, a sensing network coupled to saidgap for providing an output responsive to gap voltage condition, acutoff means operatively connected to both of said networks and havingits output operatively connected to said multivibrator to reduce itsontime, and a control means operatively connected between saidmultivibrator and said reference voltage network for conjointly varyingthe level of said reference voltage network in accordance with thefrequency setting of said multivibrator.

8. The combination as set forth in claim 7 wherein said control means isoperable to lower the level of said reference voltage at relatively lowfrequency settings of said multivibrator and further operable to raisethe level of said reference voltage at relatively high frequencysettings of said multivibrator whereby changeover is made between roughand finishing operation.

9. The combination as set forth in claim 8 wherein said multivibratorcomprises an astable multivibrator including a variable capacitoroperatively connected thereto for selectively varying the frequency ofsaid multivibrator and wherein said reference voltage network includes avariable resistor, said control means comprising a ganged operatingmeans operatively connected between said variable capacitor and saidresistor.

10. The combination as set forth in claim 9 wherein said multivibratorincludes a pair of transistors rendered alternately conductive andnonconductive and wherein said cutoff switch comprises a thirdtransistor having its control electrode and one of its principalelectrodes each connected to different one of said networks and itsother principal electrode coupled to the control electrode of one ofsaid multivibrator transistors for controlling its conduction andthereby controlling the ontirne of said pulses.

1. In an electrical discharge machining apparatus for machining anelectrically conductive workpiece by passing electrical dischargesacross a dielectric coolant filled gap between a tool electrode and theworkpiece, an electronic output switch and a power supply operativelyconnected across said gap, a pulser means operatively connected to saidswitch for operating it with pulses of selectively variable on-off timeand frequency, wherein the improvement comprises a circuit for reducingthe power content of said pulses responsive to gap abnormal condition,said circuit comprising a variable reference voltage network, a sensingnetwork operatively connected to said gap for sensing a parameterrepresentative of gap condition and providing a voltage outputrepresentative thereof, a cutoff means operatively connected to bothsaid networks and operable to reduce the ontime of said pulserresponsive to a predetermined difference between said voltages, andmeans operatively connected between said pulser means and said referencevoltage network for varying the magnitude of said reference voltage inresponse to the frequency preset on said pulser.
 2. In an electricaldischarge machining apparatus for machining an electrically conductiveworkpiece by passing electrical discharges across a dielectric coolantfilled gap between a tool electrode and the workpiece, an electronicoutput switch and a power supply operatively connected across said gap,a pulser means operatively connected to said switch for operating itwith pulses of selectively variable on-off time and frequency, whereinthe improvement comprises a circuit for reducing the power content ofsaid pulses responsive to abnormal gap voltage condition, said circuitincluding a variable reference voltage network, a sensing networkcoupled to one terminal of said gap for sensing gap voltage andproviding a voltage output representative thereof, a cutoff meansoperably connected to the output of both said networks and operative toreduce the on-time of said pulser responsive to the relative values ofsaid voltages, and means operably connected between said pulser and saidcutoff means for varying the magnitude of said reference voltage inaccordance with the frequency setting of said pulser means.
 3. In anelectrical discharge machining apparatus for machining an electricallyconductive workpiece by passing electrical discharges across adielectric coolant filled gap between a tool electrode and theworkpiece, an electronic output switch and a power operatively connectedto said gap, a presettable multivibrator operatively connected to saidelectronic switch for operating it with pulses of selectively variableand on-off time frequency, wherein the improvement comprises aprotective circuit for reducing the on-time of said pulses responsive togap abnormal condition, said circuit comprising a variable referencevoltage network, a sensing network coupled to one terminal of said gapfor providing an output responsive to gap voltage condition, a cutoffswitch operatively connected to both of said networks and having itsoutput operatively connected to said multivibrator to reduce itson-time, and a control means operatively connected between saidmultivibrator and said reference voltage network for varying the levelof said reference voltage network responsive to the frequency setting ofsaid multivibrator.
 4. The combination as set forth in claim 3 whereinsaid control means is operable to lower the level of said referencevoltage at relatively low frequency settings of said multivibrator andto raise the level of said reference voltage at relatively highfrequency settings of said multivibrator.
 5. The combination as setforth in claim 3 wherein said multivibrator comprises an astablemultivibrator including a variable capacitor operatively connectedthereto for selectively varying the frequency of said multivibrator andwherein said reference voltage network includes a variable resistor,said control means comprising a ganged operating means operativelyconnected between said variable capacitor and said resistor.
 6. Thecombination as set forth in claim 5 wherein said multivibrator includesa pair of transistors rendered alternately conductive and nonconductiveand wherein said cutoff switch comprises a third transistor having itscontrol electrode and one of its principal electrodes each connected toa different one Of said networks, and its other principal electrodecoupled to the control electrode of one of said multivibratortransistors for controlling its conduction.
 7. In an electricaldischarge machining apparatus for machining an electrically conductiveworkpiece by passing electrical discharges across a dielectric coolantfilled gap between a tool electrode and the workpiece, an electronicoutput switch and a power supply operatively connected to said gap, apresettable multivibrator operatively connected to said electronicswitch for operating it with pulses of selectively variable frequencyand on-off time, wherein the improvement comprises the protectivecircuit for reducing the on-time of said pulses responsive to gapshort-circuit condition, said circuit comprising a variable referencevoltage network, a sensing network coupled to said gap for providing anoutput responsive to gap voltage condition, a cutoff means operativelyconnected to both of said networks and having its output operativelyconnected to said multivibrator to reduce its on-time, and a controlmeans operatively connected between said multivibrator and saidreference voltage network for conjointly varying the level of saidreference voltage network in accordance with the frequency setting ofsaid multivibrator.
 8. The combination as set forth in claim 7 whereinsaid control means is operable to lower the level of said referencevoltage at relatively low frequency settings of said multivibrator andfurther operable to raise the level of said reference voltage atrelatively high frequency settings of said multivibrator wherebychangeover is made between rough and finishing operation.
 9. Thecombination as set forth in claim 8 wherein said multivibrator comprisesan astable multivibrator including a variable capacitor operativelyconnected thereto for selectively varying the frequency of saidmultivibrator and wherein said reference voltage network includes avariable resistor, said control means comprising a ganged operatingmeans operatively connected between said variable capacitor and saidresistor.
 10. The combination as set forth in claim 9 wherein saidmultivibrator includes a pair of transistors rendered alternatelyconductive and nonconductive and wherein said cutoff switch comprises athird transistor having its control electrode and one of its principalelectrodes each connected to different one of said networks and itsother principal electrode coupled to the control electrode of one ofsaid multivibrator transistors for controlling its conduction andthereby controlling the on-time of said pulses.